This invention relates generally to nondestructive testing, and more particularly to a method for determining the temperature of a test object by ultrasonically measuring the temperature of an ultrasonic delay line in close thermal contact with the test object. Furthermore, this invention relates to the temperature compensation of ultrasonic test measurements using the aforementioned ultrasonic temperature measurement of the test object.
Nondestructive testing devices can be used to inspect test objects to identify and analyze flaws and defects in the objects both during and after an inspection. Nondestructive testing allows an operator to place a probe at or near the surface of the test object in order to perform testing of both the object surface and underlying structure. Nondestructive testing can be particularly useful in some industries, e.g., aerospace, power generation, and oil and gas recovery and refining, where object testing must take place without removing the object from surrounding structures, and where hidden defects can be located in areas that would otherwise not be identifiable through visual inspection.
One example of nondestructive testing is ultrasonic testing. When conducting ultrasonic testing, an ultrasonic pulse can be emitted from a probe and passed through a test object at the characteristic sound velocity of that particular material. The sound velocity of a given material depends mainly on the modulus of elasticity, temperature and density of the material. Application of an ultrasonic pulse to a test object causes an interaction between the ultrasonic pulse and the test object structure, with sound waves being reflected back to the probe. The corresponding evaluation of the signals received by the probe, namely the amplitude and time of flight of those signals, can allow conclusions to be drawn as to the internal quality and properties of the test object (e.g., thickness) without destroying it.
Generally, an ultrasonic testing system includes a probe for sending and receiving signals to and from a test object, a probe cable connecting the probe to an ultrasonic test unit, and a screen or monitor for viewing test results. The ultrasonic test unit can include power supply components, signal generation, amplification and processing electronics, and device controls used to operate the nondestructive testing device. Some ultrasonic test units can be connected to computers that control system operations, as well as test results processing and display. Electric pulses can be generated by a transmitter and can be fed to the probe where they can be transformed into ultrasonic pulses by ultrasonic transducers. Ultrasonic transducers often incorporate piezoelectric materials which can be electrically connected to a pulsing-receiving unit in the form of an ultrasonic test unit. Portions of the surfaces of the piezoelectric materials can be metal coated, forming electrodes that can be connected to the ultrasonic test unit. During operation, an electrical waveform pulse is applied to the electrodes of the piezoelectric material causing a mechanical change in dimension and generating an acoustic wave that can be transmitted through a material such as metal or plastic to which the ultrasonic transducer is coupled. Conversely, when an acoustic wave reflected from the material under inspection contacts the surface of the piezoelectric material, it generates a voltage differential across the electrodes that is detected as a receive signal by the ultrasonic test unit or other signal processing electronics.
The amplitude, timing and transmit sequence of the electrical waveform pulses applied by the pulsing unit can be determined by various control means incorporated into the ultrasonic test unit. The pulse is generally in the frequency range of about 0.5 MHz to about 25 MHz, so it is referred to as an ultrasonic wave from which the equipment derives its name. As the ultrasonic pulses pass through the object, various pulse reflections called echoes occur as the pulse interacts with internal structures within the test object and with the opposite side (backwall) of the test object. The echo signals can be displayed on the screen with echo amplitudes appearing as vertical traces and time of flight or distance as horizontal traces. By tracking the time difference between the transmission of the electrical pulse and the receipt of the electrical signal and measuring the amplitude of the received wave, various characteristics of the material can be determined. Thus, for example, ultrasonic testing can be used to determine material thickness or the presence and size of imperfections within a given test object.
The temperature of a test object impacts both the speed at which an ultrasonic pulse travels through that object and the relative size of that object due to thermal expansion. This limits the ability of the ultrasonic testing system in certain applications, such as determining pipe corrosion rates, as the required degree of accuracy in the thickness measurements is extremely high. Compensating for thermal changes in the test object is currently a manual process requiring calibration of the system based on the temperature of the test object as measured by a thermocouple or pyrometer.
In some applications requiring continuous online monitoring, e.g., as is sometimes required in various plant or refining environments, several to several thousand probes may be positioned throughout a given facility to provide periodic or continuous monitoring of pipe conditions, including the determination of pipe corrosion rates and the identification of specific locations in need of routine maintenance to avoid pipe failures. In conjunction with the ultrasonic probes, a network of thermometers or pyrometers is required in current applications to provide temperature readings of the inspection targets. This added network of thermometers or pyrometers increases both the complexity of the ultrasonic testing system and the costs associated with installing and maintaining such a system.
It would be advantageous to perform ultrasonic measurements of a test object without requiring separate and costly devices for measuring the temperature of the test object. It would also be advantageous to perform ultrasonic measurements of a test object by automatically compensating for the effects of thermal expansion and thermally induced ultrasonic velocity changes on that test object.